(436d) Structure and Rheology In a Model Non-Polar Clay Nanocomposite

The rheology of polymer-clay nanocomposite materials is strongly affected by flow-induced changes in fluid structure. For the purposes of elucidating rheological characteristics particularly associated with the dispersed clay, recent research has demonstrated the virtues of studying idealized model materials employing polymer matrices that are essentially Newtonian in the absence of clay. In this work, we follow up on recent studies of Mobuchon and coworkers [Rheol Acta 46:1405 (2007)] on an organically modified clay (Cloisite® 15A, Southern Clay Products) dispersed in a Newtonian polybutene matrix, studying a similar sample using in situ small-angle x-ray scattering to study flow-induced orientation of the clay particles. In this work we have studied a sample with 2 wt% loading of clay. This sample was studied in an annular cone and plate x-ray capable shear cell that allows interrogation of the degree and direction of particle orientation in the flow-velocity gradient plane. Particle orientation was observed to progressively increase with increasing shear rate. Upon cessation of shear flow, orientation was found to partially relax, to an extent which depended on the previous shear rate: higher residual particle orientation was found after cessation from higher shear rates. These trends appear to be closely connected with evolution of the sample's viscoelasticity observed in mechanical rheology following low cessation. Orientation and rheology are also compared for other transient shear flow protocols, such as step-change in shear rate and shear flow reversal.